1. Field of the Invention
The present invention relates to an atomic oscillator, and in particular to a passive-type atomic oscillator of an optical pumping system.
Recently, digital networking of information has been advanced, whereby a clock source with high accuracy/high stability becomes indispensable. While an atomic oscillator such as a rubidium atomic oscillator draws attention as the clock source, downsizing/slimming is expected for mounting form on a system.
2. Description of the Related Art
FIG. 7 schematically shows a rubidium atomic oscillator having a light-microwave resonator as known in the prior art.
This atomic oscillator is composed of a pumping light source 16, a cylindrical cavity resonator 40 having light passage holes (apertures) 15a and 15b for receiving a pumping light from the light source 16, a doughnut-shaped dielectric 41 contained in the resonator for downsizing the cavity resonator 40, a gas cell 42 for enclosing rubidium atoms further contained in the dielectric 41, a light detector 14 for detecting the pumping light passing through the gas cell 42, a frequency control circuit 17 for detecting the output of the light detector 14 and for obtaining a fixed frequency, an antenna 43 for inputting a microwave from the frequency control circuit 17 and for exiting the microwave within the cavity resonator 40, a tuning screw 44 for tuning the resonance frequency of the cavity resonator 40 to the resonance frequency of the rubidium atom, a temperature control circuit 19 for keeping a temperature fixed by detecting the temperature of the gas cell 42 with a thermal element 21 such as a thermistor and by controlling a current which flows through a heater resistor 18, and a transistor 20 controlled by the temperature control circuit 19.
In operation, when the microwave cavity resonator 40 is excited with 6834.682 . . . MHz that is the resonance frequency of the rubidium atom from the frequency control circuit 17 through the antenna 43, the rubidium atoms within the gas cell 42 absorb the light received from the pumping light source 16. This phenomenon can be confirmed by the output decrease of the light detector 14.
Accordingly, the frequency control circuit 17 controls the above-mentioned microwave frequency excited by the microwave cavity resonator 40 to the microwave frequency by which the output of the light detector 14 decreases, whereby an output signal of a frequency with high stability synchronized with the resonance frequency of the rubidium atom can be obtained.
In such a prior art example, the cavity resonator 40 easily available has been used since the dielectric 41 containing the gas cell 42 is required to be provided within the resonator 40. In order to realize downsizing the cavity resonator 40, various attempts have been made, and devices such as a change of an accessible resonance mode and a high dielectric material charge have been performed.
In the prior art example shown in FIG. 7, by using a basic mode of the cylindrical cavity resonator TE111, and by having a built-in alumina ceramic dielectric 41, the cavity resonator 40 of 16 mm in diameter and 25 mm in length is realized. By utilizing this cavity resonator 40, a rubidium atomic oscillator of 23 mm (95 cc) in thickness (height) is on the market.
However, the market demands further downsizing and cost-reduction. It is difficult for the atomic oscillator using the prior art cavity resonator as mentioned above to meet the market demands as follows:
In order to meet the market demands, a microwave resonator which is substituted for the cavity resonator requiring a large space is necessary. As one example, a rubidium atomic oscillator (18 mm in thickness) using xe2x80x9chalf coaxial resonatorxe2x80x9d has begun to be offered from foreign manufacturers.
However, since a mechanism accuracy of this half coaxial resonator directly influences the resonance frequency, it is natural that a frequency adjustment mechanism should be added. For this reason, the structure of the mechanism becomes complicated and the price becomes expensive.
Also, the adjustment of the resonance frequency is necessary, and the cost increases in proportion to adjustment man-hours etc. Furthermore, in order to excite the resonator, a mechanical antenna or a probe becomes necessary, so that the mechanism becomes complicated even in this point, which causes a cost increase.
It is accordingly an object of the present invention to provide an inexpensive atomic oscillator of an optical pumping system, enabling downsizing, and excluding resonance frequency adjustments, antenna, and probe.
FIG. 1 is a diagram showing an electromagnetic field distribution in a well-known slot line. A metal conductor 2 is formed (metallized) on a high dielectric substrate 1. If the metal conductor 2 is peeled (removed) by a certain slit to form a slot line 3, electric fields concentrate on the edge of the metal conductor 2 of the ground potential so that a transmission line is formed. The electromagnetic field distribution forms a magnetic field line 4 and an electric field line 5, which forms a mode similar to a basic mode of a square waveguide, TE10.
On the other hand, a microstrip line is frequently used in a circuit of a microwave band. This is because a line section structure is simple, and also, since the ground conductor is arranged on the backside of the dielectric in which much of the electromagnetic field is distributed inside, a distribution characteristic becomes small, a passage loss is little, and a crosstalk or the like is relatively little so that the integration is easy.
A microwave resonator using such a microstrip line has been already realized. However, since it is characterized in that the magnetic field does not influence the outside as mentioned above, the application thereof to the atomic oscillator is difficult.
On the contrary, the electromagnetic field of the slot line is distributed in a wide area as mentioned above, and has a feature that the dispersion characteristic is large. This means that the passage loss is large, and unnecessary coupling of a crosstalk or the like is required to be prevented, so that it is difficult to use the slot line for a transmission line.
However, from another viewpoint, xe2x80x9capplications of atomic oscillator to microwave resonatorxe2x80x9d, there are found many advantages in the slot line as follows:
{circle around (1)} xe2x80x9cDispersion characteristic is largexe2x80x9dxe2x86x92Magnetic coupling with atoms is easy.
{circle around (2)} xe2x80x9cTE wavexe2x80x9dxe2x86x92Since only the distribution of the magnetic field exists along a line axis (direction of propagation), it becomes possible to widely secure an optical pumping area.
{circle around (3)} xe2x80x9cMaking MMIC (or MMICization) is easyxe2x80x9dxe2x86x92Since a resonance frequency is basically determined by the length of the slot line, it is possible to make the resonance frequency adjustment-free.
{circle around (4)} xe2x80x9cCoupling with a different kind of line is easyxe2x80x9dxe2x86x92Since coupling with a microstrip line or the like is easy, MMICization including an input/output coupling circuit can be easily realized.
In the present invention, a resonator using a slot line as a microwave resonator is arranged in the portion where atoms are excited, thereby enabling an atomic oscillator downsized/slimmed, and low-cost, not requiring a resonance frequency adjustment to be realized.
FIG. 2 shows an arrangement of a resonator using a slot line. In this slot line resonator 10, an upper surface of the dielectric substrate 1 is preferably metallized with the metal conductor 2. The surface of the metal conductor 2 is peeled to form the slot line 3 of e.g. xe2x80x9cWxe2x80x9d in width and xcexs/2 in length. It is to be noted that xcexs indicates 1 wavelength corresponding to a resonance frequency 6834.682 . . . MHz of e.g. the rubidium atom calculated from an rms dielectric constant on the slot line.
Also, a microstrip line 6 passing through the center of the slot line 3 and forming an open edge at a distance of e.g. xcexm/4 from the slot line 3 is provided on the backside of the dielectric substrate 1 so as to be orthogonal to each other. It is to be noted that xcexm indicates 1 wavelength corresponding to a resonance frequency 6834.682 . . . MHz of e.g. the rubidium atom calculated from the rms dielectric constant on the microstrip line 6.
If a microwave is inputted from the microstrip line 6, coupling of the electromagnetic field arises at a cross junction (intersection) between the microstrip line 6 and the slot line 3, and the microwave having propagated through the microstrip line 6 is now propagated to the slot line 3.
This electromagnetic field coupling is adapted to have a preferable size so as to perform an efficient coupling at 6834.682 . . . MHz that is the resonance frequency of the rubidium atom, and the slot line 3 is set to resonate with the frequency. The electromagnetic field distribution at this resonance assumes the magnetic field line 4 and the electric field line 5 as shown in FIG. 3.
Thus, it is possible to make the structure of the slot line resonator 10 slimmed, almost dependent on the thickness of the dielectric 1.
A container (gas cell) in which the atoms are enclosed is mounted on the slot line resonator 10. The slot line resonator 10 and the container are covered with a metallic case having a pumping light passage hole and a photo element, thereby enabling a slimmed atomic oscillator to be obtained.
Also, a container made of the same material as the above-mentioned dielectric substrate 1, having a pumping light passage hole, and enclosing therein the atoms may be formed with the slot line resonator 10 in one unit.
Also, the above-mentioned microstrip line may be provided on a backside of the container or on another printed board, and the slot line resonator is formed of the microstrip line and the slot line by mounting the container on the printed board.
Furthermore, it is preferable that the inside of the above-mentioned container is metallized with a metal conductor, a glass coating is applied to the surface, and a chemical reaction between an electromagnetic wave shield and the atoms is suppressed.
Furthermore, a glass container whose outer surface except the above-mentioned slot line and a pumping light passage hole is metallized with a metal conductor may be mounted on a printed board, and the microstrip line may be formed on a backside of the printed board.
A heater resistor for heating may be patterned around the above-mentioned metallized container.
The above-mentioned dielectric may comprise e.g. alumina ceramic.
For the above-mentioned atom, rubidium or cesium may be used.